The present invention generally relates to automated blood pressure monitoring. More specifically, the present invention relates to automated blood pressure monitors that utilize multiple data processing techniques to process oscillometric data to generate multiple oscillometric waveforms of various properties that can be selected or combined to create a blood pressure measurement that compensates for noise.
The sphygmomanometric class of automated blood pressure monitors employs an inflatable cuff to exert controlled counter-pressure on the vasculature of a patient. One large class of such monitors, exemplified by that described in U.S. Pat. Nos. 4,349,034 and 4,360,029, both to Maynard Ramsey, III and commonly assigned herewith and incorporated by reference, employs the oscillometric methodology.
In accordance with the Ramsey patents, an inflatable cuff is suitably located on the limb of a patient and is pumped up to a predetermined pressure above the systolic pressure. The cuff pressure is then reduced in predetermined decrements, and at each level, pressure fluctuations are monitored. The resultant arterial pulse signals typically consist of a DC voltage with a small superimposed variational component caused by arterial blood pressure pulsations (referred to herein as “oscillation complexes” or just simply “oscillations”).
After suitable filtering to reject the DC component and amplification, peak amplitudes of the oscillations above a given base-line are measured and stored. As the cuff pressure decrementing continues, the peak amplitudes will normally increase from a lower level to a relative maximum, and thereafter will decrease. These amplitudes form an oscillometric envelope for the patient. The lowest cuff pressure at which the oscillations have a maximum value has been found to be representative of the mean arterial pressure (MAP) of the patient. Systolic and diastolic pressures can be derived either as predetermined fractions of the oscillation size at MAP, or by more sophisticated methods of processing of the oscillation complexes.
The step deflation technique as set forth in the Ramsey patents is the commercial standard of operation. A large percentage of clinically acceptable automated blood pressure monitors utilize the step deflation rationale. When in use, the blood pressure cuff is placed on the patient and the operator usually sets a time interval, typically from 1 to 90 minutes, at which blood pressure measurements are to be repeatedly made. The noninvasive blood pressure (NIBP) monitor automatically starts a blood pressure determination at the end of the set time interval.
FIG. 1 illustrates a simplified version of the oscillometric blood pressure monitor described in the aforementioned Ramsey patents. In FIG. 1, the arm 100 of a human subject is shown wearing a conventional flexible inflatable and deflatable cuff 101 for occluding the brachial artery when fully inflated. As the cuff 101 is deflated using deflate valve 102 having exhaust 103, the arterial occlusion is gradually relieved. The deflation of cuff 101 via deflate valve 102 is controlled by central processor 107 via control line 108.
A pressure transducer 104 is coupled by a duct 105 to the cuff 101 for sensing the pressure therein. In accordance with conventional oscillometric techniques, pressure oscillations in the artery create small pressure changes in the cuff 101, and these pressure oscillations are converted into an electrical signal by transducer 104 and coupled over path 106 to the central processor 107 for processing. In addition, a source of pressurized air 109 is connected via a duct 110 through an inflate valve 111 and a duct 112 to the pressure cuff 101. The inflate valve 111 is electrically controlled through a connection 113 from the central processor 107. Also, the deflate valve 102 is connected by duct 114 via a branch connection 115 with the duct 112 leading to cuff 101.
During operation of the apparatus illustrated in FIG. 1, air under pressure at about 8-10 p.s.i. is typically available as the source of pressurized air 109. When it is desired to initiate a determination of blood pressure, the central processor 107 furnishes a signal over path 113 to open the inflate valve 111. The deflate valve 102 is closed. Air from the source 109 is communicated through inflate valve 111 and duct 112 to inflate the cuff 101 to a desired level, preferably above the estimated systolic pressure of the patient. Central processor 107 responds to a signal on path 106 from the pressure transducer 104, which is indicative of the instantaneous pressure in the cuff 101, to interrupt the inflation of the cuff 101 when the pressure in the cuff 101 reaches a predetermined initial inflation pressure that is above the estimated systolic pressure of the patient. Such interruption is accomplished by sending a signal over path 113 instructing inflate valve 111 to close. Once inflate valve 111 has been closed, the blood pressure measurement can be obtained by commencing a deflate routine.
Actual measurement of the blood pressure under the control of the central processor 107 using the deflate valve 102 and the pressure transducer 104 can be accomplished in any suitable manner such as that disclosed in the aforementioned patents or as described below. At the completion of each measurement cycle, the deflate valve 102 can be re-opened long enough to relax the cuff pressure via exhaust 103. Thereafter, the deflate valve 102 is closed for the start of a new measurement cycle.
Accordingly, when a blood pressure measurement is desired, the inflate valve 111 is opened while the cuff pressure is monitored using the pressure transducer 104 until the cuff pressure reaches the desired level. The inflate valve 111 is then closed. Thereafter, the deflate valve 102 is operated using signal 108 from microprocessor 107 and the blood pressure measurement taken.
FIG. 2 illustrates a pressure versus time graph illustrating a conventional cuff step deflation and measurement cycle for a conventional NIBP monitor. As illustrated, the cuff is inflated to an initial inflation pressure 117 above the systolic pressure 119, and the cuff is then step deflated by a pressure step 121 to the next pressure level. A timeout duration “d” is provided at each step during which the signal processing circuitry searches for oscillation complexes in accordance with the techniques described in the afore-mentioned commonly assigned patents or as described below. At the end of timeout duration “d”, the cuff pressure is decremented even if no oscillation complex is detected. This process of decrementing the pressure and searching for oscillation complexes is repeated until systolic, MAP, and diastolic pressure values can be calculated from the oscillometric envelope 116 data. The entire blood pressure determination process is then repeated at intervals set by the user, some other predetermined interval, or manually.
As shown in FIG. 2, the patient's arterial blood pressure forms an oscillometric envelope 116 comprised of a set of oscillation amplitudes 123 measured at the different cuff pressures. From the oscillometric envelope 116, systolic, MAP and diastolic blood pressures are typically calculated. However, as noted in the afore-mentioned patents, it is desired that all artifact data be rejected from the measured data so that oscillometric envelope 116 contains only the desired amplitude data and no artifacts, thereby improving the accuracy of the blood pressure determinations.
Generally, conventional NIBP monitors of the type described in the afore-mentioned patents use oscillation amplitude matching at each pressure level as one of the ways to discriminate good oscillations from artifacts. In particular, pairs of oscillations are compared at each pressure level to determine if they are similar in amplitude and similar in other attributes, such as shape, area under the oscillation curve, slope, and the like. If the oscillations compare within predetermined limits, the average amplitude and cuff pressure are stored and the pressure cuff is deflated to the next pressure level for another oscillation measurement. However, if the oscillations do not compare favorably, the first oscillation is typically discarded and another fresh oscillation is obtained. The monitor, maintaining the same pressure step, uses this newly obtained oscillation to compare with the one that was previously stored. This process normally continues until two successive oscillations match or a time limit is exceeded.
As discussed above, non-invasive blood pressure algorithms provide a blood pressure value at the end of the determination, which is then displayed to a user. However, during some blood pressure determinations, it is difficult to get data of high enough quality to enable an accurate blood pressure output. As an example, data gathered for the calculation of blood pressure could be corrupted from motion artifacts caused by the patient or by vibrations caused during transport. In the presence of such motion artifacts, signal-processing techniques that are beneficial for handling one type of artifact may not be desirable or may even be detrimental for other types. During the calculation of the blood pressure, it is difficult to determine which processing technique may be best. Therefore, it is desirable to provide a processing technique that utilizes multiple data processing strategies and then judiciously selects the best, resulting in an optimal blood pressure measurement.